1. Field of the Invention
This invention relates generally to determining geological properties of subsurface formations and more particularly to Nuclear Magnetic Resonance ("NMR") apparatus and methods for logging wellbores, particularly for obtaining information relating to the presence of hydrocarbons in such formations.
2. Background of the Art
A variety of techniques have are utilized in determining the presence and estimation of quantities of hydrocarbons (oil and gas) in earth formations. These methods are designed to determine formation parameters, including among other things, the resistivity, porosity and permeability of the rock formation surrounding the wellbore drilled for recovering the hydrocarbons. Typically, the tools designed to provide the desired information are used to log the wellbore. Much of the logging is done after the well bores have been drilled. More recently, wellbores have been logged while drilling of the wellbores, which is referred to as measurement-while-drilling "MWD" or logging-while-drilling ("LWD").
One recently evolving technique involves utilizing Nuclear Magnetic Resonance (NMR) logging tools and methods for determining, among other things porosity, hydrocarbon saturation and permeability of the rock formations. The NMR logging tools are utilized to excite the nuclei of the of the liquids in the geological formations surrounding the wellbore so that certain parameters such as spin density, longitudinal relaxation time (generally referred to in the art as "T.sub.1 ") and transverse relaxation time (generally referred to as "T.sub.2 ") of the geological formations can be measured. From such measurements, porosity, permeability and hydrocarbon saturation are determined, which provides valuable information about the make-up of the geological formations and the amount of extractable hydrocarbons.
The NMR tools generate a constant or near constant static magnetic field in a region of interest surrounding the wellbore. A radio frequency (RF) signal is induced in the formation to generate an alternating magnetic field which is perpendicular or near perpendicular to the static magnetic field in the region of interest to perturb the nuclei. The perturbation is measured to determine the parameters of interest.
The NMR logging tools typically utilize a magnetic source such as specially configured permanent magnets to generate the desired static magnetic field and one or more coils to generate the corresponding perpendicular RF magnetic field in the region of interest around the borehole.
Examples of prior art NMR logging tools and methods can be found in the following U.S. Pat. No. 3,508,438 (Alger et al.), U.S. Pat. No. 3,617,867 (Herzog), U.S. Pat. No. 3,667,035 (Slichter), U.S. Pat. No. 4,350,955 (Jackson), U.S. Pat. No. 4,467,642 (Givens), U.S. Pat. No. 4,528,508 (Vail), U.S. Pat. No. 4,560,663 (Nicksic et al.), U.S. Pat. No. 4,629,986 (Clow et al.), U.S. Pat. No. 4,710,713 (Strikman), U.S. Pat. No. 4,713,881 (Givens), U.S. Pat. No. 4,933,638 (Kenyon et al.), U.S. Pat. No. 4,717,876 (Masi et al.), U.S. Pat. No. 5,212,447 (Paltiel), U.S. Pat. No. 5,280,243 (Miller), U.S. Pat. No. 5,309,098 (Coates, et al.) and U.S. Pat. No. 5,517,115 (Prammer).
Most of the above-noted and other prior art logging tools utilize relatively complex permanent magnet and RF signal coils combinations. Furthermore, most of these tools are designed for wireline logging, i.e, for logging boreholes after they have been drilled. The formation evaluation MWD tools must operate in much harsher environments compared to wireline logging tools. This is primarily because the MWD tools are disposed in the drill string above the drill bit and, thus, must withstand high radial and axial vibrations. Furthermore, MWD tools rotate as the drill string rotates, which in prior art NMR tools tends to cause the static magnetic field to vary in the zone of interest, thereby decreasing signal to noise ratio and thus, the quality of the measurements. The Miller patent discloses an NMR apparatus that is designed for MWD applications. However, the disclosed apparatus still utilizes relatively complex magnet and coil structures.
Many of the prior art logging tools tend to utilize magnets to impart magnetic field that extends radially outward from the longitudinal axis of the tool and the borehole, and loop antennas for generating RF magnetic field which is perpendicular (orthogonal) to the static magnetic field. These types of structures tend to lead to relatively complex designs whose static magnetic field is affected due to the rotation of the tool during MWD applications. Prior art tools also usually require large areas of non-conducting material around the RF antennas. These non-conducting materials generally have a lower mechanical strength than conducting materials, a drawback in MWD applications.
The present invention addresses some of the above-noted problems with the prior art NMR logging tool and provides NMR logging tools wherein the RF antennas are linear, i.e., along the tool longitudinal direction, and one or more permanent magnets disposed along the tool longitudinal axis, primarily with their north and south poles along the tool longitudinal direction.